RE: Current draw of starter

["John Fielding" <johnf@domain.elided>]

Original post from Paul Irvine AD V8 #608


Date: Fri, 29 Mar 2002 13:46:18 -0800
From: "Paul Irvine" <pi@domain.elided>
Subject: Re: Current draw through ignition switch

To calculate the current draw of the starter you can measure the resistance
of the motor windings - for SPICA you need to measure the resistance of the
cold start solenoid as well and calculate the parallel resistance which
would be 1/(1/R1 + 1/R2). The current draw is then calculated as
12volts/resistance. For a DC starter motor you need to derate the switch 50%
so multiply the current by 1.5 and get a switch of that rating. This of
course is assuming the absence of any relays which, after all is the point
of the pushbutton starter switch - to simplify the circuit and get rid of
all of the superfluous gadgetry that will inevitably eventually cause
trouble.

Paul Irvine - Antioch CA
Project Alfa - http://home.pcmagic.net/pi



Hi Paul,

the current drawn by the starter is not simply the volts applied and the
armarture DC resistance!

The rotating armarture in the magnetic field induces a "back-EMF" in the
armarture windings which negates part of the applied voltage, so reducing
the effective applied voltage from the battery.  Hence the current drawn is
quite a bit less than expected.  Also the voltage applied is quite a bit
lower than 12V due to the resistance of the starter leads and the internal
resistance of the battery.  See the data below where the manufacturers test
data specifies voltages around 9V as being typical when cranking.

The only time the current will be the product of applied voltage and DC
resistance is during a "locked-rotor" condition because the armarture is not
rotating and supplying the "back-EMF", these are the second set of figures
noted for each starter.  This will be at a much lower battery voltage - as
specified in the manufacturers data.  A typical 12V lead-acid battery has
some internal resistance which causes the terminal voltage to drop -
sometimes dramatically when the battery is cold - and this causes the output
voltage to drop to around half under heavy discharge conditions.

The figures I have for the Alfetta starters are as follows:

116.00.05.030.09
Paris - Rhone D8E145
Normal running test - rpm = 1600-1700
Voltage 9.3V
Current = <230A
Locked rotor test
Voltage 7.2V
Current = <410A

116.08.05.030.03
Ducellier DmE124P1
Normal running test - rpm = no data given
Voltage 9.7V
Current = <230A
Locked rotor test
no data given


116.00.05.030.10
Magnetti - Marelli E95-0.9/12
Normal running test - rpm = >1800 rpm
Voltage 9.8V
Current = <220A
Locked rotor test
Voltage 7.4V
Current = <460A


116.08.05.030.00
Bosch 0,001.211.207 EF -12V-0,7CV
Normal running test - rpm = >1450
Voltage 10V
Current = <230A
Locked rotor test
Voltage 8V
Current = <400A

105.12.05.030.03
Bosch 0.001.311.110 GF-12V-1,1PS
Normal running test - rpm = 1300 (min)
Voltage 9V
Current = 290A (max)
Locked rotor test
Voltage 6V
Current = 520A (max)

The normal running  figures are for a simulated cranking mode - performed on
a dummy motor with a dynamometer brake applied.  The locked rotor test apply
a high braking force to the dummy flywheel so it remains stationary.

I hope this clarifies the situation.

John

Durban
South Africa

Alfetta 1.8L turbo

--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided